One of the toughest things to get through the heads of folks is that I can have a motor with failure modes present in it and it will still power up and run. This misconception leads to distrust of test results in otherwise skilled maintenance personnel who aren’t that well versed in motor testing. One of the tests most often overlooked when performed on a seemingly good motor is resistance to ground (RTG).

The prevailing theory is that if you truly have a failure mode related to resistance to ground that the motor wouldn’t start. This isn’t always the case. The low threshold for acceptable resistance to ground is typically 100 megaohms for motors below 1000 volts. That’s a lot of resistance. It’s not however a magic number, below which motors fail to function. You can have RTG measurements of 50-60 megaohms and the motor will start and run. If this value has significantly degraded or it’s much lower than similar motors in the same area, even these meg-ohm values can indicate a serious developing problem. IEEE recommends that for motor starting that the operational voltage, expressed as a meg-ohm value doubled plus 1000 is the minimum meg value for starting. For a 480V motor that would mean .480Meg x 2= .960 Meg + 1000, so a minimum ground resistance for starting a 480V is 1.960 Meg or round up to 2 meg-ohms.

The RTG values are an indication of both the health and cleanliness of the motor insulation and the insulation integrity of the circuit and cabling. As insulation ages, it dries and becomes embrittled. Starting and operational stresses cause insulation to crack, and openings in the insulation develop in the form of small holes. In any operating environment, contamination collects on the motors windings. As these small fissures develop in the insulation, contamination and moisture can make their way into these openings to the winding conductors themselves. This penetration provides a path for current out of the windings, and since current follows the path of least resistance, it will follow these alternate paths to get between turns, coils, phases and to ground. When it affects ground paths, we read lower than optimal RTG readings.

A low RTG reading indicates that I have alternate lower resistance paths to ground for current in the motor windings. This is a failure mode, even though the motor may in fact start and run. In the earliest stages, low RTG readings can indicate that the motor windings need to be cleaned, dipped and baked, which corrects the insulation failures. If left alone however, these failure modes can lead to catastrophic failure of the motor, which will require much more correction than a simple clean dip and bake operation.

The first step once a low RTG reading is detected is determine its point of origin. This is accomplished by isolating the motor. Assuming your RTG test was performed at the starter, go to the motor and disconnect the motor leads, and test the motor directly. If the reading is still low at the motor, that’s where the problem lies. If not, isolate and test the motor cabling. If the problem is in the motor circuit, there’s more troubleshooting to be done to isolate that root cause.

Bottom line; don’t discount a low RTG reading just because the motor starts and runs. Push further, find the location of the fault, and save the motor.